Two Novel Methods to Realize the Superplastic Forming of Ultrahigh Strength Steels

Two novel methods of obtaining microduplex structures, ferrite plus spherical carbides, in ultrahigh strength steels (~2000MPa) are introduced. One is through an adequate deformation just below the austenite-ferrite equilibrium transformation temperature (i.e. Ae3 temperature, ~983K) followed by water quenching. The adequate deformation directly leads to the formation of a (ferrite plus spherical carbides) microduplex structure. The microstructure evolution during the deformation includes pearlite transformation, cementite spheroidization and ferrite recrystallization. The other is through an adequate deformation above Ae3 temperature (~1003K) followed by water quenching to produce martensite firstly and then obtain a (ferrite plus spherical carbides) microduplex structure during warm deformation of martensite. Microstructural analysis on the microduplex structure shows that submicron carbides are located at ferrite grain boundaries while nanometer ones are dispersed inside ferrite grains. This kind of carbide distribution may suppress the coarsening of ferrite grains and form a dynamic equilibrium of ferrite grain size on a specific deformation condition. The strain rate sensitivity of the (ferrite plus spherical carbides) microduplex structures is about 0.4 at 973K and strain rate of 10-4s-1.

Mechanical Properties and Fracture Characteristics of Ultra-Microduplex Structure in High Carbon Steel

The ultra-microduplex structure was fabricated in the high carbon steel with a fully pearlitic structure after severe plastic deformation. The sizes of ferrite grains and cementite particles were about 0.4 μm and 0.1~0.2 μm, respectively. The mechanical properties of the ultra-microduplex structure were investigated using mini-tensile tests and the morphologies of fracture surfaces were observed with scanning electron microscopy (SEM). The results show that the tensile strength of the ultra-microduplex structure and the lamellar pearlite are almost at the same level, but after warm deformation, the yield strength was obviously increased and correspondingly, the elongation and the reduction of area were 19.2%, 32.1%, respectively, which are markedly higher than those of the lamellar pearlite. The tensile fracture of the ultra-microduplex structure is typical ductile fracture, however the fracture of original lamellar pearlite appears a mixture of cleavage fractures and quasi cleavage fractures.

Microstructure Evolution of Pearlitic Lamella Impacts at Ultra-High Strain Rates

The microstructure evolution of eutectoid steel with lamellar pearlite was investigated by SEM and TEM during ultra-high strain rate loading. The results indicate that ultrafine microduplex structure (ferrite + cementite) with the grain size to sub-micrometer level was observed at the surface of eutectoid steel after single pass ultra-high strain rate loading. Equiaxed ferrite grain was about 0.8 μm and the cementite lamella was spheroidized fully, and the diameter of the cementite particle was about 50 nm. The bent or fractures can occur at the edge of shock wave. Ultra-high strain rate shocking induced severe plastic deformation at the surface of materials and the cementite lamella has better plastic deformation capacity.

Microstructure Characteristics of Eutectoid Pearlitic Steel after Equal Channel Angular Pressing at Room Temperature

An ultrafine microduplex structure was successfully produced through equal channel angular pressing (ECAP) at ambient temperature. According to the morphological characteristics of cementite lamellas, the deformed pearlite consists of shear break lamella, locally thinned lamella and irregularly bent lamella. The proportion of locally thinned lamella and irregularly bent lamella has increased with the pass number of ECAP. In addition, the deformed pearlitic ferrite became a supersaturated solid solution of carbon due to the partial dissolution of cementite during ECAP.

Role of Heavy Deformation in Thermomechanical Processing on the Formation of Ultrafine-Grained Structure in Steels

The formation of ultrafine-grained structure in steels by various thermomechanical processings is reviewed from a metallurgical point of view. In the recent new type TMCP, ultrafine ferrite grains with a grain size of about 1μm are obtained when the austenite is heavily deformed at lower temperatures. In this case, dynamic phenomena such as dynamic recrystallization become prominent in the process. In the aging after heavy cold rolling of supersaturated matrix phase in two-phase alloys, the competition between the recovery or recrystallization of matrix phase and the precipitation of second phase occurs, resulting in various types of two-phase structures including microduplex structure. Microduplex structure is also obtained by annealing after heavy cold rolling of coarse two-phase structure in duplex stainless steel and high carbon steel. Recently, various severe plastic deformation processings, in which very large plastic strain over 4 is applied to the materials, have been developed to produce ultrafine grained materials with nanocrystalline and/or submicrocrystalline structures.

Superplasticity in Ultrahigh Carbon (1.6 Pct C) Steel

Ultrahigh carbon steel containing 1.6 wt pct C was processed to create microduplex structure consisting of fine-spheroidized carbides and fine ferrite grains. Elongation-to-failure tests were conducted at strain rates from 10-4s-1 to 15×10-4s-1, and at temperatures from 600 °C to 850 °C. The steel exhibited superplasticity at and above 700 °C when testing at a strain rate of 10-4s-1, and at 800 °C when testing at strain rates of 7×10-4s-1 and slower. The grains retained the equiaxed shape and initial size during deformation; dynamic grain growth was not observed after superplastic deformation, whereas carbide coarsening was observed. It is concluded that the fine ferrite grains or austensite grains are stabilized by the grain boundary carbides, and grain-boundary sliding controlled by grain boundary diffusion is the principal superplastic deformation mechanism at temperatures in the range of 700-850 °C.

Formation of (Ferrite+Cementite) Microduplex Structure by Warm Deformation in High Carbon Steels

The microstructure change by warm deformation in high-carbon steels with different initial ferrite (α) + cementite (θ) duplex microstructures has been examined. Three kinds of initial structures, i.e., pearlite, α+spheroidized θ and tempered martensite, were prepared using Fe-0.8C-2Mn and Fe-1.0C-1.4Cr alloys and compressed by 30-75% at 973K at a strain rate of 5x10-4 s-1. Equiaxed fine α grains, approximately 2μm in diameter and mostly bounded by high-angle boundaries, are formed with spheroidized θ by dynamic recrystallization during compression of the pearlite by 75%. When the (α+θ) duplex structure containing spheroidized θ was deformed, the original α grains become elongated and only subgrains are formed within them by dynamic recovery. For the tempered martensite, equiaxed α grains similar to those in the deformed pearlite were obtained after 50% compression. This indicates that the critical strain needed for the completion of dynamic recrystallization of α is smaller for the tempered martensite than for the other structures.